It is known that plasma generated by radio frequency (RF) is used in a plasma processor such as semiconductor producing equipment and electronic device producing equipment, and in a plasma generator such as a CO2 laser beam machine. It is also known that plasma is generated by CW (continuous wave) driving or pulse driving a radio frequency power source (RF power source).
FIG. 20 schematically illustrates driving of a plasma load, using the radio frequency power source (RF power source). In FIG. 20, a pulse output delivered from the radio frequency power source (RF power source) 100 is supplied to the plasma load 102, such as a plasma processor and a CO2 laser beam machine.
The pulse output from the RF power source according to the pulse driving corresponds to a radio frequency (RF) output that repeats on and off at high frequencies. In the pulse driving of the radio frequency power source (RF power source), forward wave voltage of the pulse output is intermittently supplied to the plasma load, causing repetition of plasma on and off in the plasma load.
When the RF output is switched between the on-state and the off-state, a reflected wave is generated, even though the pulse driving state is normal. During a period from the point when the RF output is switched from the off-state to the on-state, to the point when the plasma is ignited, reflection coefficient Γ becomes approximately one (Γ≈1), and this may cause a mismatching state, leading to a total reflection state for a while. At this moment, a reflected wave is generated from the plasma load toward the RF power source. Natural oscillation of a matching box, an action of plasma ignition, or the like, may be a factor of the reflected wave occurrence.
In the plasma igniting operation, the reflected wave according to the pulse driving as described above is generated in any of the following states where; plasma is in the state normally ignited, and plasma is in the unignited state without being ignited normally.
If the reflected wave thus generated enters the RF power source, high voltage of the reflected wave may cause damage of an RF power amplifier element provided in the RF power source.
In order to prevent such element damage due to the reflected wave, it is known that generation of the reflected wave that returns from the load to the RF power source is detected, and an output from the RF power source is made to droop or suspended on the basis of the detection of the reflected wave.
As a technique for detecting the unignited state of plasma, being a factor of generating the reflected wave, it is known that a power monitor monitors the reflected wave returning from the load to the RF power source, and when a peak value of the reflected wave is lower than a threshold, it is determined that the plasma is ignited, whereas when the peak value is higher than the threshold, it is determined that plasma is not ignited (see Patent Document 1).
In addition, in order to protect a high-frequency oscillator against the reflected wave, it is known that when the reflected wave exceeds a defined value, a radio frequency output is made to droop or suspended (see Patent Document 2), and forward wave power is reduced (made to droop) on the basis of a deviation between the reflected wave power and preset reflected wave power (see Patent Document 3).